Patch by Just van Rossum that changes how we search for submodules of

Patch by Just van Rossum that changes how we search for submodules of
frozen packages.  (I *think* this means that we can now have a
built-in module bar that's a submodule of a frozen package foo, by
registering the built-in module with a name "foo.bar" in the table of
builtin modules.)